1. Field of the Invention
The present invention relates to a distance measuring device for measuring a distance from the measuring device to an object by emitting light toward the object and detecting light reflected therefrom.
2. Description of the Prior Art
A distance measuring device having a semiconductor position sensitive light detector (hereinafter referred to as "PSD") has been extensively used particularly in autofocusing cameras. FIG. 1 schematically shows such a distance measuring device. The distance measuring device includes a PSD 10, a signal extraction circuit 20 connected to the PSD 10, and a distance computing circuit 30 connected to the signal extraction circuit 20. The PSD 10 is a silicon semiconductor having a 3-layer structure consisting of a P-layer, an I-layer and an N-layer. The PSD 10 receives a light spot on a light receiving surface and produces photocurrents I.sub.1 and I.sub.2 from output electrodes 11 and 12, respectively, depending on the position of the light spot incident on the light receiving surface. The signal extraction circuit 20 extracts wanted signal components from the currents I.sub.1 and I.sub.2 and cancels unwanted components contained in the currents I.sub.1 and I.sub.2, such as noise. The distance computing circuit 30 computes a distance from the measuring device to an object to be measured based on the extracted signal outputted from the signal extraction circuit 20. In the following description, a distance from the measuring device to an object to be measured or photographed will be simply referred to as "distance" unless mentioned specifically to refer to another distance.
The output electrodes 11 and 12 are provided at both ends of the P-layer of the PSD 10, and a common electrode is provided to the N-layer at a position apart by an equidistance from the output electrodes 11 and 12. A predetermined bias voltage V.sub.B is applied to the common electrode so that the P-N junction of the PSD 10 is operated in a reverse-biased condition. The P-layer serves as a light receiving surface upon which a light spot is incident. When the light is incident thereupon, the P-layer produces a photo-current I.sub.0 according to a photo-electric conversion effect. The photo-current I.sub.0 is divided into photo-currents I.sub.1 and I.sub.2. Specifically, the photo-current I.sub.1 is given by I.sub.0 .times.{L.sub.2 /(L.sub.1 +L.sub.2)} and the photo-current I.sub.2 is given by I.sub.0 .times.{L.sub.1 /(L.sub.1 +L.sub.2)}. The distance L.sub.1 is from the light spot incident position on the surface of the P-layer to the output electrode 11, and the distance L.sub.2 is from the light spot incident position to the output electrode 12. The thus divided photocurrents I.sub.1 and I.sub.2 are applied to the signal extraction circuit 20.
The distance measuring device has a light emitting diode LED which emits a light spot toward the object 90 to be photographed. The LED is disposed apart by a predetermined distance from the PSD 10. The P-layer of the PSD 10 receives the light spot reflected from the object 90 (hereinafter referred to as "reflection light spot"). With such an arrangement, a principle of trigonometrical survey is applied to measure the distance. More specifically, there is a proportional geometrical relationship between the distance and the position of the reflection light spot incident upon the P-layer of the PSD 10, which position being defined by L.sub.1 and L.sub.2. Therefore, the distance can be computed if a current ratio of the photo-current I.sub.1 to I.sub.2 is given. This computation is performed by the distance computing circuit 30.
Because the P-layer of the PSD 10 has an area larger than the area upon which the reflection light spot is incident, the photo-currents I.sub.1 and I.sub.2 derived from the output electrodes 11 and 12 contain not only the reflection light spot but also background light and noise components. If the distance measurements are performed based simply on the photo-currents I.sub.1 and I.sub.2, the accuracy of the measured distance is lowered due to the inclusion of the background light and the noise components.
In light of the above, the noise cancellation circuit as shown in FIG. 2 is provided in the signal extraction circuit 20. Although FIG. 2 indicates the noise cancellation circuit coupled to the output electrode 11, the same circuit is coupled to the output electrode 12.
The noise cancellation circuit includes a buffer circuit having an operational amplifier A.sub.1 which is d.c. biased with a bias voltage supply V.sub.R1, and a PMOSFET Q.sub.1. The buffer circuit d.c. biases the output electrode 11 to the voltage of the bias voltage supply V.sub.R1 and also outputs an 10 amplified photo-current I.sub.1 to a node x. As shown in FIG. 2, the node x is connected to the collector of an NPN transistor Q.sub.2, the base of an NPN transistor Q.sub.3, and the non-inverting input terminal of an operational amplifier A.sub.2. A current mirror circuit configured by PNP transistors Q.sub.4 and Q.sub.5 is connected to the collector of the transistor Q.sub.3. A series-connection of diodes D.sub.1 and D.sub.2 is connected to the collector of the transistor Q.sub.5. A voltage V.sub.01 developed across the diodes D.sub.1 and D.sub.2 is applied to the distance computing circuit 30. The operational amplifier A.sub.2 has an inverting input terminal applied with a reference voltage V.sub.R2, and an output terminal connected to the base of the transistor Q.sub.2 through a switch SW. A capacitor C is connected between the base of the transistor Q.sub.2 and ground.
In operation, the light emitting diode LED is turned OFF for a predetermined period of time .tau.. During the period of time the LED is OFF, the switch SW is closed to measure the background light. In this condition, a voltage V.sub.X corresponding to the light intensity of the background light appears on the node x. The operational amplifier A.sub.2 performs subtraction of the reference voltage V.sub.R2 from the voltage V.sub.X, so that the capacitor C is charged with a voltage V.sub.h (=V.sub.X -V.sub.R2) corresponding to the light intensity of the background light. It should be noted that the reference voltage V.sub.R2 corresponds to noises generated from the PSD 10 and the operational amplifier A.sub.2 is provided for removing the noise component from the voltage V.sub.X appearing at the node x.
After expiration of the predetermined period of time .tau., the switch SW is opened and the light emitting diode LED is turned ON, whereupon reception of the reflection light spot is performed for another predetermined period of time .tau.. The PSD 1 receives the reflection light spot together with the background light. The photo-current I.sub.1 generated from the PSD 10 represents a sum of the background light and the reflection light spot. Because the voltage V.sub.h corresponding to the background light has been held in the capacitor C, the current corresponding to the background light flows in ground through the transistor Q.sub.2. The voltage V.sub.X developed at the node x increases by a voltage corresponding to the reflection light spot. As a result, the current flowing in the transistor Q.sub.5 of the current mirror circuit also increases depending on the voltage increase at the node x, and the voltage across the series-connection of the diodes D.sub.1 and D.sub.2 becomes the voltage corresponding to the light intensity of the light spot.
As described above, the voltage V.sub.h corresponding purely to the background light is held in the capacitor C in advance. When the reflection light spot is received, the background light component contained in the reflection light spot is removed to obtain the voltage V.sub.01 representative of the components of only the reflection light spot. Because the same noise cancellation circuit is connected to the output electrode 12, the background light component is canceled out from the photo-current I.sub.2 outputted from the output electrode 12 to obtain the voltage V.sub.02 representative of the components of only the reflection light spot. The voltages V.sub.01 and V.sub.02 are in proportional relation to the incident position of the reflection light spot (i.e., L.sub.1 and L.sub.2). Based on these voltages V.sub.01 and V.sub.02, the distance computing circuit 30 implements a predetermined computation to obtain the distance.
The voltage corresponding to the reflection light spot is represented by: EQU V.sub.01 =(kT/q)ln{(h.sub.fe1 .multidot..DELTA.I.sub.L1 +I.sub.L1)/I.sub.S }(1)
where q is an electron charge; k, Boltzmann constant; T, an absolute temperature; I.sub.L1, a collector current of the transistor Q.sub.2 when only the background light is received; .DELTA.I.sub.L1 is a base current of the transistor Q.sub.3 when the reflection light spot is incident; h.sub.fe1 is a current amplification factor of the transistor Q.sub.3 ; and I.sub.S is a saturation current of the diodes D.sub.1 and D.sub.2.
Similar to the voltage V.sub.01, the voltage V.sub.02 produced by the noise cancellation circuit connected to the output electrode 12 is represented by: EQU V.sub.02 =(kT/q)ln{(h.sub.fe2 .multidot..DELTA.I.sub.L2 +I.sub.L2)/I.sub.S }(2)
Accordingly, the difference between these two voltages V.sub.0d (=V.sub.01 -V.sub.02) is given by: EQU V.sub.0d =(2kT/q)ln{(h.sub.fe2 .multidot..DELTA.I.sub.L2 +I.sub.L2)/I.sub.S }/ (h.sub.fe1 .multidot..DELTA.I.sub.L1 +I.sub.L1)} ()
Normally there is a relationship of h.sub.fei .multidot..DELTA.I.sub.Li &gt;I.sub.Li, and it is assumed that h.sub.fe1 .about.h.sub.fe2. Thus, the following relationship is obtained: EQU V.sub.0d .about.(2kT/q)ln(.DELTA.I.sub.L2 /.DELTA.I.sub.L1) (4)
Thus, the voltage difference V.sub.0d is determined by a logarithmic value of a ratio of two photo-currents changes. By performing a reverse logarithmic computation of the voltage difference V.sub.0d with the distance computing circuit 30, the distance can be obtained.
While the prior art is constructed to detect the light spot with excellent accuracy based on the principle described above, the following problems hinder effective detection of the light spot.